I read a few posts of people putting a tank in a cold room and how to keep it at 78 or so degrees. The responses were rule of thumb was 3 watts per gallon can raise the temp 10 degree from room temp.

I'm looking to put a 110 or 125 gallon tank in a room at home. Winter, the room fluctuates from 60-70 degrees depending if we're sleeping or awake. Very rarely is it ever colder than that in the room.

So, i'd be looking at being able to raise the temp about 20 degrees or so, does this mean on a 110 gallon tank, I should have 750 watts in heaters to be safe? Then how are things affected with Metal Hallide lights or are they?

Just want to be sure it won't be an issue keeping the tank temp stable in a room where the temp fluctuates.

I run a lot of heater power in my 125, because it's in a room that stays in the lower 60s. Overkill, probably, but I'm running 800W of heater power and the temperature has been rock solid. I have the 2 heaters set ~1 degree apart from one another as well.

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~Neilan
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Don't forget to calculate your total water volume to include any external filters/sumps/refugee's you may have connected, or plan on connecting in the future. The other thing to consider is how much heat loss your aquarium can suffer... (open top? how about your filters?) When setting your temperature, calculate any heat generated by your light fixtures as well.

I have a 350w on my 90g, around 100g including external filters. My aquarium is in a basement, which I don't provide any heat to from 10pm-5pm weekdays, and 10pm-9am weekends. It keeps a stable temperature, and the heater comes on only for around 5-10 minutes per hour. It was coming on constantly previously, but since I put glass covers over most of the aquarium, and put a lid on my sump I suffer from little heat loss (and water evaporation).